PEP 409 introduced support for the raise exc from None construct to
allow the display of the exception context to be explicitly suppressed.
This PEP retains the language level changes already implemented in PEP 409,
but replaces the underlying implementation mechanism with a simpler approach
based on a new __suppress_context__ attribute on all BaseException
instances.

PEP 409 changes __cause__ to be Ellipsis by default. Then if
__cause__ is set to None by raise exc from None, no context or cause
will be printed should the exception be uncaught.

The main problem with this scheme is it complicates the role of
__cause__. __cause__ should indicate the cause of the exception not
whether __context__ should be printed or not. This use of __cause__ is
also not easily extended in the future. For example, we may someday want to
allow the programmer to select which of __context__ and __cause__ will
be printed. The PEP 409 implementation is not amenable to this.

The use of Ellipsis is a hack. Before PEP 409, Ellipsis was used
exclusively in extended slicing. Extended slicing has nothing to do with
exceptions, so it's not clear to someone inspecting an exception object why
__cause__ should be set to Ellipsis. Using Ellipsis by default for
__cause__ makes it asymmetrical with __context__.

A new attribute on BaseException, __suppress_context__, will
be introduced. Whenever __cause__ is set, __suppress_context__
will be set to True. In particular, raise exc from cause
syntax will set exc.__suppress_context__ to True. Exception
printing code will check for that attribute to determine whether
context and cause will be printed. __cause__ will return to its
original purpose and values.

There is precedence for __suppress_context__ with the
print_line_and_file exception attribute.